Abstract: This paper analyzes the characteristics of dry-type composite film, commonly used glue, production control points and the poor opening of the composite bag, composite film Common quality problems and solutions for dry composite such as opaqueness, reduced transparency of composite film, poor peel strength, and air bubbles.
In soft package industry, complex methods are dry compound (dry? Lamination), wet lamination (wet lamination), extrusion lamination (extrusion lamination), and co-extruded composite (coextru-lamination) and the like. Among them, dry lamination refers to a method of manufacturing a composite film that does not contain volatile substances (such as solvents, etc.) when laminated with the adhesive used between the composite materials.
In the flexible packaging industry, we choose different compounding methods based on different product characteristics, costs, or material combinations. At present, the relatively extensive application in China's market is still dry compounding, which is also one of the compounding methods with relatively long application time.
1. Features of dry compound
1. Advantages and disadvantages of dry compounding and extrusion compounding
Dry compound is suitable for the production of compound films with many varieties and small production volume, suitable for most product requirements, and high production cost. Under normal process conditions, the peel strength (plastic / plastic compound) is generally N / 15mm , and its commonly used adhesives Adhesives, environmental pollution caused by solvent evaporation, and safe operation and labor hygiene problems, are also prone to solvent residues. The thickness uniformity of the film depends on the quality of the selected substrate and cannot be adjusted, but the production operation is easy and the technical requirements for workers Not high, the process technology does not change much; extrusion compounding is most suitable for large-scale continuous production, the production cost is relatively low, and the peel strength (plastic / plastic compounding) is generally 0.7 ~ 2N / 15min under general process conditions , suitable for General light packaging products. Extrusion compound basic water-based coating agent, the coating amount is very little without solvent residue, the hygienic performance is good, the thickness uniformity and average thickness of the film can be adjusted online, but the production operation is more complicated, and the technical requirements of the workers are higher and need The production process is often adjusted, and there are problems of high ambient temperature and sometimes smoke generation.
The advantages of co-extrusion compound: no glue, low cost, no organic solvent discharge, environmental protection; disadvantages: material restrictions, paper-plastic, aluminum-plastic cannot be used, and printing between films cannot be used.
2. Dry compound commonly used glue
( 1 ) Water-based glue: At present, there are two major types of acrylic resin and polyurethane resin, this type of product is only suitable for light and dry packaging.
( 2 ) Alcohol-soluble glue: There are two types of acrylic single-component glue and polyurethane two-component glue on the market. Alcohol soluble acrylic one-component glue is mainly used for some roll film and pearlescent film, while alcohol-soluble polyurethane two-component glue Glue can be used in most ordinary plastic packaging, with low strength and a small range of use.
( 3 ) Ester-soluble polyurethane glue: It has a wide range of applications and can be used in most plastic-plastic composites, paper-plastic composites, and plastic aluminum foil composites on the market.
( 4 ) Solvent-free glue: environmental protection, no solvent residue, the glue itself has poor initial adhesion, low production cost and high production efficiency. At present, the industry is moving towards solvent-free and gradually popularized.
Second, the manufacturing principle, curing mechanism and use attention of commonly used polyurethane adhesives for dry compounding
?? The main component of the two-component polyurethane adhesive is usually a modified polyester polyol containing hydroxyl groups, and the curing agent is often a combination of polyol and isocyanate. After the two components are mixed in proportion, the -OH of the main agent and the -NCO group of the curing agent are further urethane reacted. The molecular weight of the main agent will determine the suitability of the composite process. Adhesives with small molecular weights have good coating properties and leveling properties, but low initial tack strength. Conversely, large molecular weights have good initial viscosity, but poor leveling properties will affect The transparency and flexibility of the product have an impact. The molecular weight of the main agent of the adhesive will also affect the performance index that is finally achieved after aging. Proper molecular weight is the key to the design of the main agent. The curing agent containing -NCO should be highly active and can react with alcohol, water and other active hydrogen-containing substances.
1. Standard methods and precautions for dispensing
( 1 ) The standard preparation method is to first pour the main agent into the rubber barrel, pour about 1/3 of the solvent to dilute it, stir it evenly, add the curing agent, stir while adding, and then add the remaining solvent evenly. In order to avoid man-made operation defects, some manufacturers will purchase a blender to fully mix the prepared glue.
( 2 ), matters needing attention:
â‘ The tools used to prepare the glue need to be cleaned to avoid impurities from entering.
â‘¡It needs to be placed for 15 minutes to use the glue, defoamed and initially cross-linked.
â‘¢ During the production process, use a circulation pump (glue concentration is relatively uniform) to add glue.
â‘£Quality requirements of solvents: moisture, acid and other contents (total content shall not exceed 0.05% ).
3. Key points of dry compound production control
1. In the dry lamination process, the anilox roller is usually glued with multiple sets of anilox rollers.
Net roll line number
Glue amount ( g / m 2 )
Applicable products
80
4.0 ~ 4.5
Cooking products less than 121 ℃
100
3.5 ~ 3.8
Vacuum boiled, resistant to medium, rice bag, heavy bag products
120
2.7 ~ 3.0
Ordinary light, the material thickness of less than 120 μ m products, packaging and the like zipper
150 ~ 180
1.8 ~ 2.3
Lightweight small package, two-layer composite material
2. Standard process parameters
?? â‘ Composite key parameters:
Oven temperature: 50 ~ 60 ℃; 60 ~ 70 ℃; 70 ~ 80 ℃; compound pressure roller temperature: 50 ~ 80 ℃; compound pressure: without damaging the film, the compound roller pressure should be increased as much as possible.
②Curing conditions: curing temperature: 40 ~ 55 ℃; curing time: 10 ~ 96h .
The two-component adhesive does not have the ideal bonding strength immediately after the lamination, and the product needs to be sent into the curing room to be cured at 40 ~ 55 ℃ for 10 ~ 96h , ( ordinary transparent bag 12 ~ 24h , aluminum foil bag 24 ~ 36 h , multi-layer composite cooking bag is suitable for 96 h ) , according to different glues, different curing time and temperature, the thermal energy balance of the curing chamber is very important, that is, when designing the curing chamber, the heat needs to be designed as much as possible. Around the wind, at the same time, the regular exhaust air of the aging room is also very important, enough exhaust air can reduce the curing time, and can further reduce the residual solvent.
3. Mechanical technology
The inlet temperature of the drying tunnel should be designed with a gradient temperature, not too high, the inlet temperature is too high, the glue layer is easy to skin, and the internal solvent of the glue is easily sealed to form gasification. When the solvent vaporizes to a certain level, it will break through the skin formation The crater-like ring makes the transparency of the product greatly reduced. When the hardness of a certain point of the compound rubber pressure roller is not enough or there is a depression, it will cause the point to not be pressed, forming an empty glue point, and a similar leak point will appear. If there are foreign objects on the glue roller, there will be empty glue at the pressure point of the foreign objects, that is, poor glue.
The cleanliness requirement of the overall ambient air of the compound workshop is much higher than that of other workshops. Once there is more dust in the ambient air, there is dust in the air inlet of the drying tunnel after gluing, which is easier to adhere to the surface of the adhesive layer. Foreign body spots are formed between the two base films. To reduce the problems caused by dust, it is necessary to install a high-mesh filter at the air inlet to remove or reduce the dust carried in the air flow.
Four, dry composite common quality problems and solutions
1. The opening of the composite bag is not good
( 1 ) Some components of the glue react with the inner film slip agent, which reduces the original slip of the film.
( 2 ) The amount of glue is too large, and the curing temperature is too high.
( 3 ) After aging, the bag is not completely cooled. If this situation has already occurred, blowing it through the mouth with a high-pressure air gun after bag making can effectively improve its opening.
2. The composite membrane is not dry
After the composite product is cured at a certain temperature and time, the peeling adhesive layer between the composite films still has a certain viscosity, that is, after the composite film is peeled off, the film and the film are still stuck together when they are re-attached together (normally cured) The product, after peeling, cannot be re-bonded). The impermeability of the composite film glue will directly affect the peel strength of the composite film, and the slight non-drying phenomenon, the heat seal strength is poor after the bag is made, and the layer is sticky after peeling, and the place where the heat seal is prone to wrinkles. When it is fast, after one or two hours, the bag will be edge-sealed, and it will appear late. After one week or half a month of storage, the bag will wrinkle, and even in the place where it has been bent. There will be wrinkles.
Generally speaking, there are two main reasons for the impermeability of the glue: on the one hand, the total content of the glue is exceeded, and on the other hand, it is caused by insufficient curing agent during the configuration of the glue. Both of these reasons will cause the insufficient curing dose to crosslink with the main agent. The main agent of the polyurethane adhesive is a polymer polyester and polyurethane compound terminated with —OH , which has a certain viscosity. The main agent of the polyurethane adhesive only reacts with the isocyanate in the curing agent to produce a network cross-linked structure. Higher peel strength. One water molecule can react with two —NCO groups, the molecular weight of one water molecule is 18 , and the molecular weight of one —NCO group is 42 , which means that 18g of water can react with 42 × 2 = 84g — NCO groups. Generally, the content of the curing agent of the polyurethane adhesive for dry compound is 70% , and the content of the 70% curing agent- NCO is about 12% . Then the amount of 75% of the curing agent consumed by 18g of water is 84/12% = 700g . We assume that 20 kg of ethyl acetate has a water content of 0.2% , then 20 kg of ethyl acetate contains 20 g of water . If it reacts completely, it will consume 1.55 kg of curing agent . Normal 20kg binder main agent, equipped with 4kg curing agent, if the moisture content of ethyl acetate is 0.2% , then the amount of curing agent consumed by the reaction is an incredible amount.
The solution to the phenomenon that the glue of the composite membrane is dry:
â‘ Reduce the total content of water and alcohol in ethyl acetate (the total content of water and alcohol is below 0.05% ).
â‘¡When using nylon and other films that are easy to absorb moisture, increase the proportion of curing agent ( 5% ~ 10% ). Put it into the curing room for a period of time during the "Meiyu" season, and then start production.
? â‘¢ Try to reduce the amount of residual solvent in printing ink, and strengthen the online monitoring of residual solvent.
â‘£ Shorten the storage time of the prepared adhesive. When preparing the adhesive, reduce the amount of each preparation, increase the number of times of mixing, shorten the storage time, and increase the temperature of the drying tunnel and reduce the amount of residual solvent.
3. The transparency of the composite film is reduced
( 1 ) Adhesive itself: the color of the adhesive is too dark, too dark yellow or yellow-red. After drying , this part of the darker color will be left on the film accordingly. Therefore, when the finished composite film is required to have high transparency, Choose yellowish or even colorless adhesives , choose the right two-component Adhesives, can reduce the risk of the composite end product a "mist" and even eliminating the "mist" phenomenon.
Tension Effect (2) of the substrate surface: the surface tension of the film does not meet the requirements, it can not be uniformly wetting glue, so that non-uniform drying causes poor transparency film, the surface tension of the composite film reached up to 38 possible Because of the above, if the surface tension is not high, it must be treated. The purpose of the treatment is to roughen the surface, fluff the surface, and increase the concavity of the surface area. When the adhesive is in contact with its surface, it can produce a good wetting effect. The adhesive will penetrate the groove to increase the adhesion fastness.
( 3 ). The effects of insufficient fluidity and poor flatness of adhesives, insufficient or excessive glue application, etc.
4. Poor peel strength
( 1 ) Incomplete curing. Curing completely refers to 100% curing of the hydroxyl group . However, in actual life, the impurities in the ethyl ester consume part of the -NCO group, resulting in an unbalanced ratio of the main agent and the curing agent, or the curing agent is added less, or the added curing agent and The hydroxyl groups in the ink react, resulting in insufficient curing agent, resulting in incomplete curing.
( 2 ) The wetting tension of the substrate is insufficient, so that the adhesive cannot fully wet the coated surface, resulting in poor peel strength.
( 3 ) The compatibility between glue and ink is not good.
( 4 ) Insufficient coating amount will not only generate bubbles but also reduce the peel strength, but if the coating amount is too large, the ink will fall off the printing substrate.
( 5 ) Peel strength decreases after boiling or cooking. Because BOPA is easy to absorb moisture, the strength sometimes decreases after boiling. Therefore, in addition to selecting the appropriate cooking glue, it is also necessary to consider the dry state of nylon and the actual application conditions. It is necessary to fully estimate that the moisture absorption of nylon will cause the peel strength to decrease.
5. Generate bubbles
( 1 ) Bubbles caused by insufficient glue application. Insufficient amount of glue is a relative concept. For non-printed composite film, the amount of glue is between 1.8 ~ 2.2g / m 2 , and the best value for printed composite film is about 2.8 g / m 2 . In addition to the peel strength, the factor determining the amount of sizing is the important factor of coating leveling, and the leveling of the coating has an impact on it in addition to the amount of sizing. Surface tension and the viscosity of the glue will affect it.
( 2 ) Bubbles caused by poor drying. The upper surface of the transparent composite film is "foggy" and has poor transparency. Adjusting the drying capacity depends first on the drying system of the equipment, and secondly considers the amount of glue, the greater the amount of glue, the higher the drying capacity requirements.
( 3 ) Bubbles generated by the blocking of the screen roller. The coating roller has not been cleaned with the plate washing liquid for a long time, causing blockage. Because it is impossible to completely clean the residual glue in the anilox roller after each compounding, the residual glue is deposited and solidified at the bottom of the mesh hole day after day, and the amount of glue gradually decreases, and small bubbles are gradually generated. More, from small to larger.
( 4 ) The unevenness of the composite heat roller and the pressure roller generates bubbles, and air is introduced into the uneven areas to generate bubbles. Such bubbles periodically appear. Another important but easily overlooked reason is that the bearing parts at both ends of the pressure roller are worn or there are tiny bubbles, which are not parallel to the heat roller and can also generate bubbles.
( 5 ) The bubbles cannot be eliminated due to insufficient curing temperature. At the same temperature, the glue with large molecular weight has poor fluidity, and the temperature should be appropriately increased. The temperature of the curing chamber is not enough to eliminate bubbles, and the temperature of the composite heat roller is not enough to generate bubbles. The purpose of heating is to dissolve and flow the dry adhesive, eliminate small holes and small gaps, discharge air under pressure, and make the two-layer film Heat tightly.
( 6 ) Equipment process reasons. The reason for this kind of bubbles is that the adhesive is not completely dry, and some are caused by insufficient drying of the ink solvent, which often occurs in the area where large areas are overprinted.
Fifth, the impact of high temperature and high humidity weather on dry compound and matters needing attention
At present, most of the dry-type compound used are two-component reactive polyester and polyurethane adhesives, but its performance is generally related to the choice of isocyanate, molecular structure of polyester polyol, degree of polymerization, functional groups and mixing Different conditions are different. The two-component adhesive reaction is a cross-linking curing reaction between the main agent containing a hydroxyl component at the end of the polymer and the curing agent containing an isocyanate group component in the molecular structure, which is combined into a polyurethane with a higher compactness and containing an isocyanate group component The curing agent has a strong activity and reacts with substances containing hydroxyl groups and amine groups, and the reaction rate is more than 10 times faster than that with high molecular polyester and polyurethane main agents . This is also the reason why most polyurethane adhesive manufacturers restrict the water, alcohol and amine in the thinner (solvent) in the instructions. ?
In the dry lamination process, the impact of high humidity on the lamination film has many aspects, such as:
a . The glue is impervious to dryness, and remains sticky after the compound is cured, resulting in a decrease in peel strength and even being unusable;
b . The cross-linking speed slows down and the initial adhesion of the adhesive decreases, which can easily cause tunneling of the composite film;
c . High-temperature cooking film bags, the inner layer and bag breaking phenomenon increase during the high-temperature cooking process; the composite film is prone to spots, white spots and crystal points.
In view of the impact of high temperature and high humidity weather on dry compounding, targeted preventive measures should be taken during the production process:
①A variety of films must be controlled, such as nylon and other moisture-absorbing films. When storing, use metal aluminum foil or a film with high barrier properties to seal the nylon film on a shelf in a dry environment. "In the season, the nylon film used for high-temperature cooking products is stored in a curing room at 40 ℃ for 4 ~ 8H before compounding or printing . It is recommended to use it when it is used. The effect will be ideal. The films used cannot be stacked directly on the ground.
â‘¡ For the thinner (ethyl acetate) of the glue, quality requirements should be controlled in terms of procurement requirements. For each batch of solvent entering production, the content of moisture and alcohol must be inspected and analyzed to check whether it exceeds the standard and sample retention for future reference.
â‘¢The production workshop must control the compound environment, install an exhaust fan in the room to enhance the indoor air circulation, but pay attention to not be able to linearly blow against the flow of the compound machine, and check the guide roller, compound roller and mesh roller for water droplets at any time. Wipe it again with a dry cloth.
â‘£When configuring the adhesive, the amount of the mixture should be appropriately reduced. It should not be placed too long as it is used. At the same time, the amount of curing agent should be ensured to make it as close as possible to the upper limit. It can also be appropriately increased during the "Meiyu" season Curing dose ( 5% ~ 10% ), increase the degree of crosslinking between the main agent and curing agent.
⑤ Change the exhaust air volume of the compound machine to increase the air outlet pipe. In addition, if the air outlet pipe is long, it is advisable to install an induced draft fan at the air outlet pipe mouth, so that the air outlet speed is greater than the air inlet speed, so that the drying tunnel is formed low Air pressure is suitable for solvent volatilization to reduce residual solvent. In addition, the temperature in the drying tunnel can be adjusted as high as possible, but it should not exceed the temperature range of the film, and the temperature of the film entry area should be appropriate, not too high.
â‘¥ When compounding, pay attention to the scraper and guide roller near the glue tank at any time, whether there is water droplets, and at the same time reduce the speed and reduce the residue of the solvent. The high temperature and high humidity weather can easily accelerate the solvent volatilization speed in the glue tank, and the local temperature near the glue tank Too low, it is easy to cause water vapor condensation, pay attention to the working state of the glue circulation pump at any time during production, and regularly test the viscosity of the glue to control the glue concentration.
6. Conclusion
There are many factors that affect the quality of the dry composite process, which are generally categorized as follows: 1. Factors of the raw and auxiliary materials themselves; 2. Human operation defects and mechanical defects; 3. Hardware environment, temperature and humidity control factors; 4. Effects of the curing process factor. In view of these factors, it is not difficult to check the cause of abnormalities in the nuclear dry compound, so as to solve the problem. The key to grasping the dry compound production process is to standardize and refine each link.
Reusable Cotton Produce Bags,Cotton Mesh Produce Bags,Mesh Grocery Bag,Organic Cotton Produce Bags
Suzhou Yuli Industrial Co.,Ltd , https://www.packageyuli.com